The present invention relates to an improved apparatus for performing haemodialysis on a patient suffering from renal failure. Although the present invention can be used to perform haemodialysis on any suitable patient, the present invention is particularly, although not exclusively, suitable for use in performing haemodialysis on a neonate.
Acute renal failure is common in very small preterm babies and can sometimes be secondary to other problems such as hypotension and hypoxia caused by medical or surgical conditions. However, babies having a weight of less than 8 kg are notoriously difficult to dialyse, especially those having a weight as low as 1 kg.
Peritoneal dialysis, whereby the peritoneum in the abdomen of the patient is used as a membrane across which molecules from the blood are passed, as opposed to conventional haemodialysis, can sometimes be an easier method to perform on very small preterm babies. However, it is not always feasible and it does not always efficiently clear the molecules, which can include urea, potassium and phosphates for example, from the blood.
There are two main factors which can make haemodialysis difficult in very small preterm babies: —
A first factor is that the volumes of the extracorporeal circuits typically used in existing haemodialysis apparatuses are large relative to the blood volume of such a baby, necessitating the priming of the extracorporeal circuit with fresh or modified blood. For example, at the time of writing, the typical minimum haemodialysis extracorporeal circuit volume is around 49 ml, which is large in relation to a baby's blood volume of around 85 ml/kg. As a result of this, with conventional haemodialysis apparatus, it is preferable that the extracorporeal circuit be primed with blood for heavier babies, and essential that this is the case with very small preterm babies.
A second factor is that the rate of flow of blood from the access vessels of very small preterm babies is typically very low, resulting in inadequate haemodialysis and/or clotting within the extracorporeal circuit. The reason for this low rate of flow is that the maximum rate of flow of blood through a blood vessel is limited by, inter alia, the diameter of the blood vessel. To elaborate, for laminar flow, the rate of flow of blood can be analysed using Poiseuille's Law which states that the rate of flow of fluid is directly proportional to the pressure gradient multiplied by the fourth power of the radius of the tube through which the fluid flows. It follows that blood can only be extracted from very small babies having very narrow blood vessels at a very low flow rate for acceptable pressure gradients, when compared with larger children having larger blood vessels.